The present invention relates to the field of casting, and more particularly to a shell mold, and to methods of manufacturing and using such a shell mold.
So-called “lost-wax” or “lost-pattern” casting methods have been known since antiquity. They are particularly suitable for producing metal parts that are complex in shape. Thus, lost-pattern casting is used in particular for producing turbine engine blades.
In lost-pattern casting, the first step normally comprises making a pattern out of a material having a melting temperature that is comparatively low, such as for example out of wax or resin, and then overmolding the mold onto the pattern. After removing the material of the pattern from the inside of the mold, whence the name of such methods, molten metal is cast into the mold in order to fill the cavity that the pattern has formed inside the mold by being removed therefrom. Once the metal has cooled and solidified, the mold may be opened or destroyed in order to recover a metal part having the shape of the pattern.
In order to be able to make a plurality of parts simultaneously, it is possible to unite a plurality of patterns in a single assembly in which they are connected together by a tree that forms casting channels in the mold for the molten metal.
Among the various types of mold that can be used in lost-pattern casting, so-called “shell” molds are known that are formed by dipping the pattern or the assembly of patterns into a slip, and then dusting refractory sand onto the pattern or the assembly of patterns coated in the slip in order to form a shell around the pattern or the assembly, and then baking the shell in order to solidify the slip and thus consolidate the slip and the sand. Several successive operations of dipping and dusting may be envisaged in order to obtain a shell of sufficient thickness prior to baking it. The term “refractory sand” is used in the present context to designate any granular material of grain size that is small enough to satisfy the desired production tolerances, that is capable, while in the solid state, of withstanding the temperature of the molten metal, and that is capable of being consolidated into a single solid piece by the slip during baking of the shell.
The term “metal” is used in the present context to designate both pure metals and metal alloys, and in particular metal alloys known as monocrystalline alloys such as those developed since the end of 1970s in order to enable parts to be cast in the form of a single grain. Conventional metal alloys are equiaxed polycrystallines: in their solid state, they form a plurality of grains of substantially identical size, typically about 1 millimeter (mm), but of more or less random orientation. The boundaries between grains constitute weak points in a metal part made out of such an alloy. However, using additives to strengthen these inter-grain boundaries presents the drawback of reducing the melting point temperature, which is a disadvantage, particularly when the parts produced in this way are for use at high temperature. Typically, monocrystalline alloys are nickel alloys with a concentration of titanium and/or aluminum that is lower than 10 molar percent (% mol). Thus, after solidifying, these alloys form two-phase solids, with a γ first phase and a γ′ second phase. The γ phase presents a face-centered cubic crystal lattice, in which the nickel, aluminum, and/or titanium atoms may occupy any position. In contrast, in the γ′ phase, the aluminum, and/or titanium atoms form a cubic configuration, occupying the eight corners of the cube, while the nickel atoms occupy the faces of the cube.
One of these alloys is the nickel alloy “AM1” developed jointly by SNECMA and the ONERA laboratories, the Ecole des Mines in Paris, and IMPHY SA. Parts made of such an alloy can not only achieve mechanical strength that is particularly high along all stress axes, but can also achieve improved thermal resistance, since additives for binding the crystalline grains together more strongly may be omitted. Thus, metal parts made of such monocrystalline alloys may advantageously be used, e.g. in the hot parts of turbines.
Nevertheless, in order to benefit fully from the advantages of monocrystalline alloys in order to obtain advantageous thermomechanical properties in a part made by casting, it may be desirable to ensure that the metal undergoes directional solidification in the mold. The term “directional solidification” is used in the present context to mean that control is exerted over the nucleation and the growth of solid crystals in the molten metal as it passes from the liquid state to the solid state. The purpose of such directional solidification is to avoid the negative effects of grain boundaries within the part. Thus, the directional solidification may be columnar or monocrystalline. Columnar directional solidification consists in orienting all of the grain boundaries in the same direction so that they cannot contribute to propagating cracks. Monocrystalline directional solidification consists in ensuring that the part solidifies as a single crystal, so as to eliminate all grain boundaries.
The published specification of French patent application FR 2 874 340 describes a shell mold that is particularly adapted to implementing a casting method with directional solidification. That shell mold of the prior art includes a central cylinder extending, along a main axis, between a casting cup and a base, and a plurality of molding cavities arranged as an assembly around the central cylinder, each one connected to the casting cup by a feed channel. In order to enable directional solidification of molten metal in the molding cavities, each of them is also connected via a baffle-selector to a starter adjacent to the base. Furthermore, the shell mold also includes at least one heat shield that is substantially perpendicular to said main axis.
In a casting method using said shell mold, after casting the molten metal through the casting cup, the molten metal is cooled progressively, along said main axis from the base towards the casting cup. By way of example, this may be performed by gradually extracting the shell mold from a heater chamber, along the main axis, towards the base, while cooling the base.
Because the molten metal is cooled progressively going away from the plate, the first solid grains nucleate in the starters adjacent to the plate. The configuration of the baffle-selectors then prevents propagation of more than a single grain towards each molding cavity.
The purpose of using at least one heat shield is to try to ensure that the propagation front of the crystallization in each molding cavity remains substantially perpendicular to the main axis. A sloping propagation front would be likely to cause unwanted grains to nucleate in the molding cavity. However, it is nevertheless found to be difficult to prevent such sloping, in particular in molding cavities that are complex in shape.